I didn't have any trouble changing the ArcGIS Online URL to point at my organization.
I was able to sign in using a 'standard' ArcGIS Online account that is linked to my organization (that has a username and password)
BUT I couldn't find any option to sign in using our Enterprise managed SAML (single sign on) accounts.
Am I correct in assuming that ArcGIS Earth doesn't (yet) support this?
Thanks in advance.